The Siemens 6ES7953-8LJ30-0AA0 is a Micro Memory Card (MMC) for the S7-300, C7, and ET 200 families — the spare-part designation in the description tells you this is the replacement card for existing installations, not a new-design option. It stores 512 kbyte on Flash-EPROM, which means the program and data survive a power loss without a battery.
This card is classified as mature in its lifecycle — still available through the supply channel, but no longer the target for new machine designs. If you are keeping a line running on S7-300 or ET 200, this is the correct spare. Source it against an RFQ; we quote to order.
The 512 kbyte capacity is enough for a medium-size S7-300 program — think a few hundred network blocks with symbol tables and comments. Flash-EPROM retains data for a minimum of 10 years after the last write, and the card handles at least 100,000 write/delete cycles, so it holds up to frequent recipe or parameter updates on a packaging or bottling line.